1. Field of the Invention:
This invention relates to motorized drive devices for cameras in which a motor drives a transmission system which in turn drives a camera mechanism.
2. Description of the Related Art:
Recently, the automatic drive device has widely been used in cameras. As the charging of the shutter and the lens and the winding-up and rewinding of the film are driven by either a single motor or a plurality of motors, the manageability of the camera is being very much improved.
The use of such a device can remove the necessity of using the mechanical stopping control of the motor when the completion of winding-up or the completion of charging is electrically detected to stop the motor. But, the motor is rotating at a very high speed and so cannot stop suddenly in response to the stop signal. Hence it takes a somewhat long time for the object being driven to stop stably as measured from the moment at which the stop signal has been given. This permits the object being driven to overrun. Particularly in the case of the small speed reduction ratio for the high speed mode, the object will overrun a long distance. If it happens that the overrun distance takes a longer value than a critical level, many drawbacks will arise. For example, the film windup system will advance the film a longer length than that of one frame. On this account, during a short time before the completion of winding-up, the drive signal for the motor may be made to vary in duty fashion, or the drive voltage may be made to decrease in order to decelerate the motor so that the length through which the film is advanced is accurately adjusted to that of one frame. To assure the accurate control of deceleration of the motor, the load on the film windup system must be taken into account. When the torque to pull out the film from the cartridge increases largely, or when the voltage of the electrical power source falls below the satisfactory operating level (including the case that it is caused by a large drop of the ambient temperature), the film cannot be advanced through the full length of one frame. Therefore, it is required to always monitor the speed of movement of the film not only during the deceleration control period but also during its preceding motor drive period. It is also of importance to discriminate between an accidental large decrease of the speed of film and the film end.
However, if the same abnormal low speed (including the film stop) detecting process as in the motor drive period which precedes the deceleration control period is applied even to the deceleration control period, the following problem arises. Because the deceleration control process and the abnormal low speed detecting process must be carried out simultaneously, and, particularly when they are executed in software fashion by a microcomputer, because both processes must be treated time-serially by a timer interrupt handing, it takes a considerably long time to do this timer interrupt handling, causing the treatment of the main routine to delay. Therefore, the other systems, for example, the charge system, will suffer over-charging or other serious faulty operations with a high possibility.
Also, if the same abnormal low speed detecting process is used in both of the deceleration control period and the motor drive period that precedes it, the reference level for the detection must be set considerably low in view of the deceleration control period. Even when the speed decreases considerably in the motor drive period that precedes the deceleration control period, the automatic changeover of the speed reduction ratio from the high to the low speed will fail.